EERC Update

A Road Map for Biofuels Research-Production of Green Diesel

By Joshua R. Strege

To continue the theme from last month's column, "This is not your father's ethanol process," we would like to shift gears a bit from producing ethanol and focus on the production of synthetic, or green, diesel, which is significantly different than traditional biodiesel production. Although biodiesel production significantly trails ethanol production, momentum is growing based on technology advances and economics which could result in green diesel production exceeding ethanol production in the not-too-distant future.

It is apparent that the greatest impact biomass fuels may have on the transportation industry, on a field-to-wheels basis (miles driven per acre harvested), is when the biomass is used to produce green diesel. In the past, the greatest disadvantage for diesel fuel has been the reluctance, for a variety of reasons, of the American consumer to purchase diesel vehicles. These concerns appear to be fading, with a resurgence in diesel vehicles in the United States.

There are three primary methods, or pathways, of producing green diesel. The first is thermal gasification of biomass to syngas followed by Fischer-Tropsch conversion to green diesel. This pathway has the significant advantage of being a well-established process for natural gas feedstocks, used by Germany during World War II and then by Sasol Ltd. of South Africa, to produce a fuel that is almost completely compatible with petroleum-derived diesel. However, the product of Fischer-Tropsch synthesis contains no aromatics or cyclic compounds, which can negatively impact fuel density, resulting in an overall loss in engine performance.

Second is thermal gasification followed by methanol synthesis over a catalyst bed, followed by conversion of the methanol to dimethyl ether (DME). DME is used as an aerosol propellant and manufactured by several companies using nonrenewable methanol as a feedstock. Like diesel, DME can be used in compression ignition engines and presents a higher fuel economy than do spark-ignition fuels. Unlike diesel, DME is molecularly homogeneous, which allows engines to be precisely tuned to optimize combustion. However, DME is also a gas at room temperature and pressure, requiring it to be compressed to a liquid for transportation.

Lastly is pyrolysis followed by hydrogenation of the bio-oil product to green diesel. Pyrolysis is already practiced on the commercial scale for production of food additives. Bio-oil often contains aromatic and cyclic compounds, so hydrogenated bio-oil is likely to meet all of the existing specifications for diesel fuel without requiring any additives. The primary disadvantage to using hydrogenated bio-oil is the risk that the green diesel product may contain unacceptable amounts of aromatics or other unsaturated compounds. These compounds can lead to smog formation and limit the shelf life of the fuel. Such compounds could be eliminated by adding more hydrogen to the fuel during hydrogenation, which may negatively impact process cost and complexity.

The six pathways from biomass to transportation fuel described in our last two columns are by no means the only advanced methods being considered today, but are the most likely to find success on the large scale. Some of the processes have already found commercial success in other markets (e.g, food additives) or in using raw materials other than biomass (e.g, natural gas instead of syngas). Other processes show promise because they represent simple, efficient or low-cost options.

Whichever process or combination of processes ultimately finds the greatest success at producing transportation fuels, it is clear that the future of biofuels will have a larger and more varied market than the corn ethanol and biodiesel markets of today.

Joshua R. Strege is a research engineer at the EERC in Grand Forks, N.D. He can be reached at jstrege@undeerc.org or (701) 777-3252.